Allow parallelisation of masks (#636)

* Allow parallelisation of masks
Increasing performance improvement with increased masking complexity and edit size.

* Address comments
 - Rename Mask#clone to Mask#copy
 - Rename Mask2D#copy to Mask2D#copy2D
 - Correct formatting

* cx -> centerX

* Make various operations relying on a single SingleThreadQueueExtent instance (mainly brushes) thread safe

worldedit-core/src/main/java/com/sk89q/worldedit/EditSession.java

@@ -71,6 +71,7 @@ import com.sk89q.worldedit.function.mask.NoiseFilter2D;
 import com.sk89q.worldedit.function.mask.RegionMask;
 import com.sk89q.worldedit.function.mask.SingleBlockTypeMask;
 import com.sk89q.worldedit.function.mask.SolidBlockMask;
+import com.sk89q.worldedit.function.mask.WallMakeMask;
 import com.sk89q.worldedit.function.operation.ChangeSetExecutor;
 import com.sk89q.worldedit.function.operation.ForwardExtentCopy;
 import com.sk89q.worldedit.function.operation.Operations;
@@ -1406,15 +1407,7 @@ public class EditSession extends PassthroughExtent implements AutoCloseable {
         if (region instanceof CuboidRegion) {
             return makeCuboidWalls(region, pattern);
         } else {
-            replaceBlocks(region, new Mask() {
-                @Override
-                public boolean test(BlockVector3 position) {
-                    int x = position.getBlockX();
-                    int z = position.getBlockZ();
-                    return !region.contains(x, z + 1) || !region.contains(x, z - 1) || !region
-                        .contains(x + 1, z) || !region.contains(x - 1, z);
-                }
-            }, pattern);
+            replaceBlocks(region, new WallMakeMask(region), pattern);
         }
         return changes;
     }

worldedit-core/src/main/java/com/sk89q/worldedit/extension/factory/parser/mask/LiquidMaskParser.java

@@ -1,6 +1,6 @@
 package com.sk89q.worldedit.extension.factory.parser.mask;
 
-import com.boydti.fawe.function.mask.LiquidMask;
+import com.boydti.fawe.object.mask.LiquidMask;
 import com.google.common.collect.ImmutableList;
 import com.sk89q.worldedit.WorldEdit;
 import com.sk89q.worldedit.extension.input.ParserContext;

worldedit-core/src/main/java/com/sk89q/worldedit/extension/factory/parser/mask/XAxisMaskParser.java

@@ -19,7 +19,7 @@
 
 package com.sk89q.worldedit.extension.factory.parser.mask;
 
-import com.boydti.fawe.function.mask.XAxisMask;
+import com.boydti.fawe.object.mask.XAxisMask;
 import com.google.common.collect.ImmutableList;
 import com.sk89q.worldedit.WorldEdit;
 import com.sk89q.worldedit.extension.input.ParserContext;

worldedit-core/src/main/java/com/sk89q/worldedit/extension/factory/parser/mask/YAxisMaskParser.java

@@ -19,7 +19,7 @@
 
 package com.sk89q.worldedit.extension.factory.parser.mask;
 
-import com.boydti.fawe.function.mask.YAxisMask;
+import com.boydti.fawe.object.mask.YAxisMask;
 import com.google.common.collect.ImmutableList;
 import com.sk89q.worldedit.WorldEdit;
 import com.sk89q.worldedit.extension.input.ParserContext;

worldedit-core/src/main/java/com/sk89q/worldedit/extension/factory/parser/mask/ZAxisMaskParser.java

@@ -19,7 +19,7 @@
 
 package com.sk89q.worldedit.extension.factory.parser.mask;
 
-import com.boydti.fawe.function.mask.ZAxisMask;
+import com.boydti.fawe.object.mask.ZAxisMask;
 import com.google.common.collect.ImmutableList;
 import com.sk89q.worldedit.WorldEdit;
 import com.sk89q.worldedit.extension.input.ParserContext;

worldedit-core/src/main/java/com/sk89q/worldedit/extent/MaskingExtent.java

@@ -44,7 +44,7 @@ import static com.google.common.base.Preconditions.checkNotNull;
 public class MaskingExtent extends AbstractDelegateExtent implements IBatchProcessor, Filter {
 
     private Mask mask;
-    private LoadingCache<Long, ChunkFilterBlock> threadIdToFilter = FaweCache.IMP.createCache(() -> new CharFilterBlock(getExtent()));
+    private final LoadingCache<Long, ChunkFilterBlock> threadIdToFilter;
 
     /**
      * Create a new instance.
@@ -56,6 +56,14 @@ public class MaskingExtent extends AbstractDelegateExtent implements IBatchProce
         super(extent);
         checkNotNull(mask);
         this.mask = mask;
+        this.threadIdToFilter = FaweCache.IMP.createCache(() -> new CharFilterBlock(getExtent()));
+    }
+
+    private MaskingExtent(Extent extent, Mask mask, LoadingCache<Long, ChunkFilterBlock> threadIdToFilter) {
+        super(extent);
+        checkNotNull(mask);
+        this.mask = mask;
+        this.threadIdToFilter = threadIdToFilter;
     }
 
     /**
@@ -64,7 +72,7 @@ public class MaskingExtent extends AbstractDelegateExtent implements IBatchProce
      * @return the mask
      */
     public Mask getMask() {
-        return mask;
+        return this.mask;
     }
 
     /**
@@ -79,38 +87,40 @@ public class MaskingExtent extends AbstractDelegateExtent implements IBatchProce
 
     @Override
     public <B extends BlockStateHolder<B>> boolean setBlock(BlockVector3 location, B block) throws WorldEditException {
-        return mask.test(location) && super.setBlock(location, block);
+        return this.mask.test(location) && super.setBlock(location, block);
     }
 
     @Override
     public boolean setBiome(BlockVector2 position, BiomeType biome) {
-        return mask.test(position.toBlockVector3()) && super.setBiome(position, biome);
+        return this.mask.test(position.toBlockVector3()) && super.setBiome(position, biome);
     }
 
     @Override
     public boolean setBiome(int x, int y, int z, BiomeType biome) {
-        return mask.test(BlockVector3.at(x, y, z)) && super.setBiome(x, y, z, biome);
+        return this.mask.test(BlockVector3.at(x, y, z)) && super.setBiome(x, y, z, biome);
     }
 
     @Override
-    public IChunkSet processSet(IChunk chunk, IChunkGet get, IChunkSet set) {
-        ChunkFilterBlock filter = threadIdToFilter.getUnchecked(Thread.currentThread().getId());
-        return filter.filter(chunk, get, set, this);
+    public IChunkSet processSet(final IChunk chunk, final IChunkGet get, final IChunkSet set) {
+        final ChunkFilterBlock filter = threadIdToFilter.getUnchecked(Thread.currentThread().getId());
+        return filter.filter(chunk, get, set, MaskingExtent.this);
     }
 
     @Override
-    public void applyBlock(FilterBlock block) {
-        //TODO: Find a way to make masking thread safe without having to synchonise the whole extent
-        synchronized (this) {
-            if (!mask.test(block)) {
-                block.setOrdinal(0);
-            }
+    public void applyBlock(final FilterBlock block) {
+        if (!this.mask.test(block)) {
+            block.setOrdinal(0);
         }
     }
 
     @Override
     public Extent construct(Extent child) {
         if (child == getExtent()) return this;
-        return new MaskingExtent(child, mask);
+        return new MaskingExtent(child, this.mask.copy(), this.threadIdToFilter);
+    }
+
+    @Override
+    public Filter fork() {
+        return new MaskingExtent(getExtent(), this.mask.copy(), this.threadIdToFilter);
     }
 }

worldedit-core/src/main/java/com/sk89q/worldedit/function/mask/AbstractMask.java

@@ -19,9 +19,20 @@
 
 package com.sk89q.worldedit.function.mask;
 
+import org.jetbrains.annotations.Nullable;
+
 /**
  * A base class of {@link Mask} that all masks should inherit from.
  */
 public abstract class AbstractMask implements Mask {
 
+    @Override
+    @Nullable
+    public Mask copy() {
+        try {
+            return (Mask) super.clone();
+        } catch (CloneNotSupportedException e) {
+            return null;
+        }
+    }
 }

worldedit-core/src/main/java/com/sk89q/worldedit/function/mask/BiomeMask.java

@@ -102,4 +102,9 @@ public class BiomeMask extends AbstractMask {
         return null;
     }
 
+    @Override
+    public Mask copy() {
+        return new BiomeMask(extent, new HashSet<>(biomes));
+    }
+
 }

worldedit-core/src/main/java/com/sk89q/worldedit/function/mask/BiomeMask2D.java

@@ -98,4 +98,9 @@ public class BiomeMask2D extends AbstractMask2D {
         return biomes.contains(biome);
     }
 
+    @Override
+    public Mask2D copy2D() {
+        return new BiomeMask2D(extent, new HashSet<>(biomes));
+    }
+
 }

worldedit-core/src/main/java/com/sk89q/worldedit/function/mask/BlockCategoryMask.java

@@ -56,4 +56,9 @@ public class BlockCategoryMask extends AbstractExtentMask {
     public Mask2D toMask2D() {
         return null;
     }
+
+    @Override
+    public Mask copy() {
+        return new BlockCategoryMask(getExtent(), category);
+    }
 }

worldedit-core/src/main/java/com/sk89q/worldedit/function/mask/BlockMask.java

@@ -328,4 +328,9 @@ public class BlockMask extends ABlockMask {
         }
         return new BlockMask(getExtent(), cloned);
     }
+
+    @Override
+    public Mask copy() {
+        return new BlockMask(getExtent(), ordinals.clone());
+    }
 }

worldedit-core/src/main/java/com/sk89q/worldedit/function/mask/BlockStateMask.java

@@ -28,6 +28,7 @@ import com.sk89q.worldedit.world.block.BlockState;
 import com.sk89q.worldedit.world.block.BlockType;
 import com.sk89q.worldedit.world.block.BlockTypes;
 
+import java.util.HashMap;
 import java.util.Map;
 import javax.annotation.Nullable;
 
@@ -82,4 +83,11 @@ public class BlockStateMask extends AbstractExtentMask {
     public Mask2D toMask2D() {
         return null;
     }
+
+    @Override
+    public Mask copy() {
+        Map<String, String> statesClone = new HashMap<>();
+        states.forEach(statesClone::put);
+        return new BlockStateMask(getExtent(), statesClone, strict);
+    }
 }

worldedit-core/src/main/java/com/sk89q/worldedit/function/mask/BlockTypeMask.java

@@ -70,6 +70,12 @@ public class BlockTypeMask extends AbstractExtentMask {
         }
     }
 
+    private BlockTypeMask(Extent extent, boolean[] types, boolean hasAir) {
+        super(extent);
+        this.types = types;
+        this.hasAir = hasAir;
+    }
+
     /**
      * Add the given blocks to the list of criteria.
      *
@@ -134,4 +140,9 @@ public class BlockTypeMask extends AbstractExtentMask {
         return null;
     }
 
+    @Override
+    public Mask copy() {
+        return new BlockTypeMask(getExtent(), types.clone(), hasAir);
+    }
+
 }

worldedit-core/src/main/java/com/sk89q/worldedit/function/mask/BoundedHeightMask.java

@@ -57,4 +57,10 @@ public class BoundedHeightMask extends AbstractMask {
         return null;
     }
 
+    @Override
+    public Mask copy() {
+        // The mask is not mutable. There is no need to clone it.
+        return this;
+    }
+
 }

worldedit-core/src/main/java/com/sk89q/worldedit/function/mask/ExistingBlockMask.java

@@ -55,4 +55,10 @@ public class ExistingBlockMask extends AbstractExtentMask {
         return null;
     }
 
+    @Override
+    public Mask copy() {
+        // The mask is not mutable. There is no need to clone it.
+        return this;
+    }
+
 }

worldedit-core/src/main/java/com/sk89q/worldedit/function/mask/ExpressionMask.java

@@ -89,4 +89,9 @@ public class ExpressionMask extends AbstractMask {
         return new ExpressionMask2D(expression, timeout);
     }
 
+    @Override
+    public Mask copy() {
+        return new ExpressionMask(expression.clone(), timeout);
+    }
+
 }

worldedit-core/src/main/java/com/sk89q/worldedit/function/mask/ExpressionMask2D.java

@@ -72,4 +72,9 @@ public class ExpressionMask2D extends AbstractMask2D {
         }
     }
 
+    @Override
+    public Mask2D copy2D() {
+        return new ExpressionMask2D(expression.clone(), timeout);
+    }
+
 }

worldedit-core/src/main/java/com/sk89q/worldedit/function/mask/InverseMask.java

@@ -31,4 +31,9 @@ public class InverseMask extends AbstractMask {
     public Mask inverse() {
         return mask;
     }
+
+    @Override
+    public Mask copy() {
+        return new InverseMask(mask.copy());
+    }
 }

worldedit-core/src/main/java/com/sk89q/worldedit/function/mask/InverseSingleBlockStateMask.java

@@ -18,6 +18,12 @@ public class InverseSingleBlockStateMask extends ABlockMask {
         this.ordinal = state.getOrdinalChar();
     }
 
+    private InverseSingleBlockStateMask(Extent extent, char ordinal, boolean isAir) {
+        super(extent);
+        this.ordinal = ordinal;
+        this.isAir = isAir;
+    }
+
     @Override
     public boolean test(BlockVector3 vector) {
         int test = getExtent().getBlock(vector).getOrdinal();
@@ -40,4 +46,9 @@ public class InverseSingleBlockStateMask extends ABlockMask {
     public Mask inverse() {
         return new SingleBlockStateMask(getExtent(), BlockState.getFromOrdinal(ordinal));
     }
+
+    @Override
+    public Mask copy() {
+        return new InverseSingleBlockStateMask(getExtent(), ordinal, isAir);
+    }
 }

worldedit-core/src/main/java/com/sk89q/worldedit/function/mask/InverseSingleBlockTypeMask.java

@@ -27,4 +27,10 @@ public class InverseSingleBlockTypeMask extends ABlockMask {
     public BlockType getBlockType() {
         return BlockTypes.get(internalId);
     }
+
+    @Override
+    public Mask copy() {
+        // The mask is not mutable. There is no need to clone it.
+        return this;
+    }
 }

worldedit-core/src/main/java/com/sk89q/worldedit/function/mask/Mask.java

@@ -100,4 +100,10 @@ public interface Mask {
     default boolean replacesAir() {
         return false;
     }
+
+    /**
+     * Returns a copy of the mask. Usually for multi-threaded operation
+     * @return a clone of the mask
+     */
+    Mask copy();
 }

worldedit-core/src/main/java/com/sk89q/worldedit/function/mask/Mask2D.java

@@ -34,4 +34,6 @@ public interface Mask2D {
      */
     boolean test(BlockVector2 vector);
 
+    Mask2D copy2D();
+
 }

worldedit-core/src/main/java/com/sk89q/worldedit/function/mask/MaskIntersection.java

@@ -33,6 +33,7 @@ import java.util.Map;
 import java.util.Map.Entry;
 import java.util.Set;
 import java.util.function.Function;
+import java.util.stream.Collectors;
 import javax.annotation.Nullable;
 
 import static com.google.common.base.Preconditions.checkNotNull;
@@ -45,9 +46,9 @@ import static org.slf4j.LoggerFactory.getLogger;
  */
 public class MaskIntersection extends AbstractMask {
 
-    private final Set<Mask> masks;
-    private Mask[] masksArray;
-    private boolean defaultReturn;
+    protected final Set<Mask> masks;
+    protected Mask[] masksArray;
+    protected boolean defaultReturn;
 
     /**
      * Create a new intersection.
@@ -60,6 +61,12 @@ public class MaskIntersection extends AbstractMask {
         formArray();
     }
 
+    protected MaskIntersection(Set<Mask> masks, Mask[] masksArray, boolean defaultReturn) {
+        this.masks = masks;
+        this.masksArray = masksArray;
+        this.defaultReturn = defaultReturn;
+    }
+
     public static Mask of(Mask... masks) {
         Set<Mask> set = new LinkedHashSet<>();
         for (Mask mask : masks) {
@@ -250,4 +257,11 @@ public class MaskIntersection extends AbstractMask {
         return new MaskIntersection2D(mask2dList);
     }
 
+    @Override
+    public Mask copy(){
+        Set<Mask> masks = this.masks.stream().map(Mask::copy).collect(Collectors.toSet());
+        Mask[] maskArray = (Mask[]) Arrays.stream(this.masksArray).map(Mask::copy).toArray();
+        return new MaskIntersection(masks, maskArray, this.defaultReturn);
+    }
+
 }

worldedit-core/src/main/java/com/sk89q/worldedit/function/mask/MaskIntersection2D.java

@@ -25,6 +25,7 @@ import java.util.Arrays;
 import java.util.Collection;
 import java.util.HashSet;
 import java.util.Set;
+import java.util.stream.Collectors;
 
 import static com.google.common.base.Preconditions.checkNotNull;
 
@@ -33,7 +34,7 @@ import static com.google.common.base.Preconditions.checkNotNull;
  */
 public class MaskIntersection2D implements Mask2D {
 
-    private final Set<Mask2D> masks = new HashSet<>();
+    protected final Set<Mask2D> masks = new HashSet<>();
 
     /**
      * Create a new intersection.
@@ -97,4 +98,10 @@ public class MaskIntersection2D implements Mask2D {
         return true;
     }
 
+    @Override
+    public Mask2D copy2D() {
+        Set<Mask2D> masksCopy = this.masks.stream().map(Mask2D::copy2D).collect(Collectors.toSet());
+        return new MaskIntersection2D(masksCopy);
+    }
+
 }

worldedit-core/src/main/java/com/sk89q/worldedit/function/mask/MaskUnion.java

@@ -22,12 +22,14 @@ package com.sk89q.worldedit.function.mask;
 import com.sk89q.worldedit.math.BlockVector3;
 
 import java.util.ArrayList;
+import java.util.Arrays;
 import java.util.Collection;
 import java.util.LinkedHashSet;
 import java.util.List;
 import java.util.Map.Entry;
 import java.util.Set;
 import java.util.function.Function;
+import java.util.stream.Collectors;
 import javax.annotation.Nullable;
 
 /**
@@ -55,6 +57,10 @@ public class MaskUnion extends MaskIntersection {
         super(mask);
     }
 
+    private MaskUnion(Set<Mask> masks, Mask[] maskArray, boolean defaultReturn) {
+        super(masks, maskArray, defaultReturn);
+    }
+
     public static Mask of(Mask... masks) {
         Set<Mask> set = new LinkedHashSet<>();
         for (Mask mask : masks) {
@@ -111,4 +117,11 @@ public class MaskUnion extends MaskIntersection {
         }
         return new MaskUnion2D(mask2dList);
     }
+
+    @Override
+    public Mask copy() {
+        Set<Mask> masksCopy = masks.stream().map(Mask::copy).collect(Collectors.toSet());
+        Mask[] maskArray = (Mask[]) Arrays.stream(masksArray).map(Mask::copy).toArray();
+        return new MaskUnion(masksCopy, maskArray, defaultReturn);
+    }
 }

worldedit-core/src/main/java/com/sk89q/worldedit/function/mask/MaskUnion2D.java

@@ -22,6 +22,8 @@ package com.sk89q.worldedit.function.mask;
 import com.sk89q.worldedit.math.BlockVector2;
 
 import java.util.Collection;
+import java.util.Set;
+import java.util.stream.Collectors;
 
 /**
  * Tests true if any contained mask is true, even if it just one.
@@ -59,4 +61,10 @@ public class MaskUnion2D extends MaskIntersection2D {
         return false;
     }
 
+    @Override
+    public Mask2D copy2D() {
+        Set<Mask2D> masksCopy = masks.stream().map(Mask2D::copy2D).collect(Collectors.toSet());
+        return new MaskUnion2D(masksCopy);
+    }
+
 }

worldedit-core/src/main/java/com/sk89q/worldedit/function/mask/Masks.java

@@ -92,6 +92,11 @@ public final class Masks {
             public boolean test(BlockVector2 vector) {
                 return !mask.test(vector);
             }
+
+            @Override
+            public Mask2D copy2D() {
+                return Masks.negate(mask.copy2D());
+            }
         };
     }
 
@@ -113,6 +118,11 @@ public final class Masks {
             public Mask2D toMask2D() {
                 return mask;
             }
+
+            @Override
+            public Mask copy() {
+                return Masks.asMask(mask.copy2D());
+            }
         };
     }
 
@@ -143,6 +153,18 @@ public final class Masks {
             return this;
         }
 
+        // No need to properly clone an always true mask
+        @Override
+        public Mask copy() {
+            return new AlwaysTrue();
+        }
+
+        // No need to properly clone an always true mask
+        @Override
+        public Mask2D copy2D() {
+            return new AlwaysTrue();
+        }
+
     }
 
     protected static class AlwaysFalse implements Mask, Mask2D {
@@ -172,6 +194,18 @@ public final class Masks {
             return other;
         }
 
+        // No need to properly clone an always false mask
+        @Override
+        public Mask copy() {
+            return new AlwaysFalse();
+        }
+
+        // No need to properly clone an always true mask
+        @Override
+        public Mask2D copy2D() {
+            return new AlwaysFalse();
+        }
+
     }
 
 }

worldedit-core/src/main/java/com/sk89q/worldedit/function/mask/NoiseFilter.java

@@ -96,4 +96,9 @@ public class NoiseFilter extends AbstractMask {
         return new NoiseFilter2D(getNoiseGenerator(), getDensity());
     }
 
+    @Override
+    public Mask copy() {
+        return new NoiseFilter(noiseGenerator, density);
+    }
+
 }

worldedit-core/src/main/java/com/sk89q/worldedit/function/mask/NoiseFilter2D.java

@@ -87,4 +87,9 @@ public class NoiseFilter2D extends AbstractMask2D {
         return noiseGenerator.noise(pos.toVector2()) <= density;
     }
 
+    @Override
+    public Mask2D copy2D() {
+        return new NoiseFilter2D(noiseGenerator, density);
+    }
+
 }

worldedit-core/src/main/java/com/sk89q/worldedit/function/mask/OffsetMask.java

@@ -105,4 +105,9 @@ public class OffsetMask extends AbstractMask {
         }
     }
 
+    @Override
+    public Mask copy() {
+        return new OffsetMask(mask.copy(), offset.toImmutable());
+    }
+
 }

worldedit-core/src/main/java/com/sk89q/worldedit/function/mask/OffsetMask2D.java

@@ -92,4 +92,9 @@ public class OffsetMask2D extends AbstractMask2D {
         return getMask().test(mutable);
     }
 
+    @Override
+    public Mask2D copy2D() {
+        return new OffsetMask2D(mask.copy2D(), BlockVector2.at(offset.getX(), offset.getZ()));
+    }
+
 }

worldedit-core/src/main/java/com/sk89q/worldedit/function/mask/RegionMask.java

@@ -72,4 +72,9 @@ public class RegionMask extends AbstractMask {
         return null;
     }
 
+    @Override
+    public Mask copy() {
+        return new RegionMask(region.clone());
+    }
+
 }

worldedit-core/src/main/java/com/sk89q/worldedit/function/mask/SingleBlockStateMask.java

@@ -18,6 +18,12 @@ public class SingleBlockStateMask extends ABlockMask {
         this.ordinal = state.getOrdinalChar();
     }
 
+    private SingleBlockStateMask(Extent extent, char ordinal, boolean isAir) {
+        super(extent);
+        this.ordinal = ordinal;
+        this.isAir = isAir;
+    }
+
     @Override
     public boolean test(BlockVector3 vector) {
         int test = getExtent().getBlock(vector).getOrdinal();
@@ -50,4 +56,9 @@ public class SingleBlockStateMask extends ABlockMask {
         }
         return null;
     }
+
+    @Override
+    public Mask copy() {
+        return new SingleBlockStateMask(getExtent(), ordinal, isAir);
+    }
 }

worldedit-core/src/main/java/com/sk89q/worldedit/function/mask/SingleBlockTypeMask.java

@@ -17,6 +17,12 @@ public class SingleBlockTypeMask extends ABlockMask {
         this.internalId = type.getInternalId();
     }
 
+    private SingleBlockTypeMask(Extent extent, int internalId, boolean isAir) {
+        super(extent);
+        this.internalId = internalId;
+        this.isAir = isAir;
+    }
+
     @Override
     public final boolean test(BlockState state) {
         return state.getBlockType().getInternalId() == internalId;
@@ -35,4 +41,9 @@ public class SingleBlockTypeMask extends ABlockMask {
     public boolean replacesAir() {
         return isAir;
     }
+
+    @Override
+    public Mask copy() {
+        return new SingleBlockTypeMask(getExtent(), internalId, isAir);
+    }
 }

worldedit-core/src/main/java/com/sk89q/worldedit/function/mask/SolidBlockMask.java

@@ -35,4 +35,9 @@ public class SolidBlockMask extends BlockMask {
         return null;
     }
 
+    @Override
+    public Mask copy() {
+        return new SolidBlockMask(getExtent());
+    }
+
 }

worldedit-core/src/main/java/com/sk89q/worldedit/function/mask/WallMakeMask.java

@@ -0,0 +1,26 @@
+package com.sk89q.worldedit.function.mask;
+
+import com.sk89q.worldedit.math.BlockVector3;
+import com.sk89q.worldedit.regions.Region;
+
+public class WallMakeMask implements Mask {
+
+    private final Region region;
+
+    public WallMakeMask(Region region) {
+        this.region = region.clone();
+    }
+
+    @Override
+    public boolean test(BlockVector3 position) {
+        int x = position.getBlockX();
+        int z = position.getBlockZ();
+        return !region.contains(x, z + 1) || !region.contains(x, z - 1) || !region.contains(x + 1, z) || !region.contains(x - 1, z);
+    }
+
+    @Override
+    public Mask copy() {
+        return new WallMakeMask(region);
+    }
+
+}

worldedit-core/src/main/java/com/sk89q/worldedit/internal/expression/Expression.java

@@ -31,8 +31,10 @@ import org.antlr.v4.runtime.misc.ParseCancellationException;
 import org.antlr.v4.runtime.tree.ParseTreeWalker;
 
 import java.time.Instant;
+import java.util.HashSet;
 import java.util.List;
 import java.util.Objects;
+import java.util.Set;
 
 /**
  * Compiles and evaluates expressions.
@@ -60,19 +62,55 @@ import java.util.Objects;
  * pass values for all slots specified while compiling.
  * To query slots after evaluation, you can use the {@linkplain #getSlots() slot table}.
  */
-public class Expression {
+public class Expression implements Cloneable {
 
     private final SlotTable slots = new SlotTable();
     private final List<String> providedSlots;
     private final ExpressionParser.AllStatementsContext root;
     private final Functions functions = Functions.create();
     private final CompiledExpression compiledExpression;
+    private final String initialExpression;
 
     public static Expression compile(String expression, String... variableNames) throws ExpressionException {
         return new Expression(expression, variableNames);
     }
 
     private Expression(String expression, String... variableNames) throws ExpressionException {
+        this.initialExpression = expression;
+
+        slots.putSlot("e", new LocalSlot.Constant(Math.E));
+        slots.putSlot("pi", new LocalSlot.Constant(Math.PI));
+        slots.putSlot("true", new LocalSlot.Constant(1));
+        slots.putSlot("false", new LocalSlot.Constant(0));
+
+        for (String variableName : variableNames) {
+            slots.initVariable(variableName)
+                .orElseThrow(() -> new ExpressionException(-1,
+                    "Tried to overwrite identifier '" + variableName + "'"));
+        }
+        this.providedSlots = ImmutableList.copyOf(variableNames);
+
+        CharStream cs = CharStreams.fromString(expression, "<input>");
+        ExpressionLexer lexer = new ExpressionLexer(cs);
+        lexer.removeErrorListeners();
+        lexer.addErrorListener(new LexerErrorListener());
+        CommonTokenStream tokens = new CommonTokenStream(lexer);
+        ExpressionParser parser = new ExpressionParser(tokens);
+        parser.removeErrorListeners();
+        parser.addErrorListener(new ParserErrorListener());
+        try {
+            root = parser.allStatements();
+            Objects.requireNonNull(root, "Unable to parse root, but no exceptions?");
+        } catch (ParseCancellationException e) {
+            throw new ParserException(parser.getState(), e);
+        }
+        ParseTreeWalker.DEFAULT.walk(new ExpressionValidator(slots.keySet(), functions), root);
+        this.compiledExpression = new ExpressionCompiler().compileExpression(root, functions);
+    }
+
+    private Expression(String expression, Set<String> variableNames) throws ExpressionException {
+        this.initialExpression = expression;
+
         slots.putSlot("e", new LocalSlot.Constant(Math.E));
         slots.putSlot("pi", new LocalSlot.Constant(Math.PI));
         slots.putSlot("true", new LocalSlot.Constant(1));
@@ -147,4 +185,8 @@ public class Expression {
         functions.setEnvironment(environment);
     }
 
+    public Expression clone() {
+        return new Expression(initialExpression, new HashSet<>(providedSlots));
+    }
+
 }